2-Ethyl-4-methyl-1H-imidazol-3-ium bromide

In the title salt, the bromide ions accept N—H⋯Br hydrogen bonds from the cations, generating C(8) chains propagating in the b-axis direction.


Structure description
The unique structure of imidazole, containing two N atoms in a five-membered ring, permits it to accept a proton on one of its N atoms to form a cation and simultaneously deliver another proton from the other N atom to a suitable acceptor. In fact, this sort of shuttling action has been proposed as part of the catalytic mechanism of a number of enzymes (Mikulski & Silverman, 2010), and is consistent with the proton-conductivity properties of imidazole in the solid state where long hydrogen-bonded chains are present (Kawada et al., 1970). These moieties and their derivatives have been implicated in proton-coupled electron-transfer processes (Huynh & Meyer, 2007;Onidas et al., 2010). Consequently, there have been many theoretical (Scheiner & Yi, 1996;Kumar & Venkatnathan, 2015) and structural studies (Purdy et al., 2007;Kim et al., 2016) investigating these species. In this paper, we report a crystal structure containing the 2-ethyl-4methyl-1H-imidazol-3-ium (C 6 H 11 N 2 + ) cation. There have been four previous reports of structures containing this species (CSD refcode LEZSAL, Amanokura et al., 2007;POJFOL, Beckett et al., 2014;HOJJAT, Arici et al., 2014;UMALAX, Kazimierczuk et al., 2016).
The title salt, 1, crystallizes in the monoclinic space group P2 1 /c with one ion pair in the asymmetric unit ( Fig. 1) and consists of C 6 H 11 N 2 + cations and Br À anions. The C8 methyl group is close to coplanar with the imidazole ring [N1-C2-C7-C8 = À8.03 (15) ]. Otherwise, the metrical parameters of the cation agree well with those observed in the other structures involving this species. In the extended structure, the component ions are data reports linked by N-HÁ Á ÁBrÁ Á ÁH-N hydrogen bonds (Table 1) into C(8) (Etter et al., 1990) chains propagating in the b-axis direction. The chains are cross-linked in the c-axis direction by weak C-HÁ Á ÁBr hydrogen bonds (Fig. 2).

Synthesis and crystallization
The title compound resulted from an attempt to link two 2-ethyl-4-methylimidazole rings with a two-carbon chain by the reaction of 2-Et-4-Me-imidazole (6.20 g, 56.3 mmol) with BrCH 2 CH 2 Br (5.32 g, 28.3 mmol) in EtOH at 80 C overnight and several hours at 100 C. Ba(OH) 2 Á8H 2 O (8.95 g, 28.3 mmol) was added with ethanol and water and heated to dissolve. On cooling, the mixture was rotovapped down and extracted between water and ether, and the ether layer was evaporated down to 3.1 g of an oil identified as primarily the starting imidazole by NMR. Recovery of about half of the starting imidazole must mean that the oligomer forms preferentially over the dimer. The barium ion was removed from the water layer by titration with H 2 SO 4 followed by filtration. The solution was rotovapped down to an oil that precipitated a mass of salts on cooling. More crystals of 1 Table 1 Hydrogen-bond geometry (Å , ). Symmetry codes: (i) Àx þ 1; y þ 1 2 ; Àz þ 1 2 ; (ii) x þ 1; Ày þ 1 2 ; z þ 1 2 ; (iii) x þ 1; y; z; (iv) x; Ày þ 1 2 ; z À 1 2 .

Figure 1
The

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2.

Funding information
RJB wishes to acknowledge the ONR Summer Faculty Research Program for funding in 2019 and 2020.

2-Ethyl-4-methyl-1H-imidazol-3-ium bromide
Crystal data C 6 H 11 N 2 + ·Br − M r = 191.08 Monoclinic, P2 1 /c a = 6.8432 (6)  where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max < 0.001 Δρ max = 0.53 e Å −3 Δρ min = −0.36 e Å −3 Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes. Refinement. All hydrogen atoms were located in difference Fourier maps and those attached to N were refined isotropically. Those attached to carbon atoms were refined in idealized geometry using a riding model with with atomic displacement parameters of U iso (H) = 1.2U eq (C) [for CH 3 , 1.5U eq (C)] with C-H distances of 0.95 to 0.99 Å.